1. Field of Invention
This invention relates to apparatus for controlling the flow of power on ac power transmission lines, and in particular, a power flow controller which can control any one or more of reactive power flow, transmission line impedance, transmission line voltage magnitude, and transmission line voltage phase angle.
2. Background Information
Electric power flow through an ac transmission line is a function of the line impedance, the magnitude of the sending and receiving end voltages, and the phase angle between these voltages. To date, electric power transmission systems have been designed with the understanding that these three parameters determining the power flow cannot be controlled fast enough to handle dynamic system conditions. Furthermore, the available control means usually compensated or controlled only one of the three variables: impedance, voltage, or phase-angle. That is, transmission systems having been designed with fixed or mechanically-switched series and shunt reactive compensations, together with voltage regulating and phase-shifting transformer tap changers, to optimize line impedance, minimize voltage variation, and control power flow under steady-state or slowly changing load conditions. The dynamic system problems generally have been handled by overdesign; power transmission systems have been designed with generous stability margins to recover from worst case contingencies resulting from faults, line and generator outages, and equipment failures. This practice of overdesign, of course, has resulted in the under utilization of the transmission system.
In recent years, energy, environment, right-of-way, and cost problems have delayed the construction of both generation facilities and new transmission lines. This has necessitated a change in the traditional power system concepts and practices; better utilization of existing power systems has become imperative.
Higher utilization of power transmission systems, without an appreciable degradation in the reliability of the supply of electric power, is possible only if the power flow can be controlled rapidly during the following dynamic system disturbances.
The electric transmission system is frequently subjected to disturbances of various magnitudes. For example, some power generators or some of the parallel transmission lines may be switched out for maintenance. Large loads may be switched on and off. There may also be line to ground or line to line faults due to insulation break-downs or equipment failures. These disturbances may result in a sudden and sharp increase or decrease in the transmitted electric power. The electric power is provided by rotating generators driven by some kind of turbines which generate mechanical power. The mechanical output power of the turbines cannot be changed quickly to bring the mechanical power in balance with the new and rapidly changing electrical power demand. Consequently, the generators are forced to accelerate or decelerate. The change in rotational speed in some generators results in a corresponding angular position change with respect to the steady angular position maintained at the other end of the line by other generators. The angular position change between the sending and receiving end generators alters the amount of electric power transmitted. Once the disturbance is over (fault cleared, new transmission system configuration, new power generation level or new load demand established), the disturbed generators try to assume a new angular position appropriate to the new steady-state condition of the power system. However, the generators together with the associated turbines have significant rotational inertia and, for this reason, the new angular position is usually reached only after an "overshoot" or oscillation. These transient angular changes and oscillations, of course, manifest themselves as transient electric power changes and oscillations. In the extreme case, these transient changes cannot be stabilized; the equilibrium between the available mechanical power and transmitted electric power cannot be reestablished and the angular "overshoot" keeps increasing (that is, the machine keeps accelerating) until the generator is shut down. It also can happen that the angular oscillation remains unchanged, or even grows, due to insufficient damping of the power system. Ultimately, this would also result in a power system shut down.
The ability of a power system to provide electric power to meet load demand is indicated by the term "stability". The term stability means that the generators of an electric power system tend to run in synchronism. The term "transient stability" means that a power system can recover normal operation following a major disturbance (fault, loss of generation, etc.). The term "dynamic stability" means that a power system can recover normal operation following a minor disturbance that initiates power oscillation. In other words, a dynamically stable power system has positive damping.
In the last fifteen years, considerable efforts have been expended in the development of fast, thyristor-controlled equipment for the dynamic compensation and control of ac electric power transmission systems. Again, this thyristor-controlled equipment addressed one of the three power system parameters which determine power flow: voltage, impedance, and phase-angle. Thus, thyristor-controlled static var compensators, thyristor-controlled series compensators, and thyristor-controlled phase-shifting transformers have been or are being developed for the control of transmission line voltage (achieved by the control of reactive power flow), line impedance, and phase-angle.
Thyristor-controlled, static var compensators are used to control indirectly the transmission line voltage, and thereby the transmitted electric power, by generating reactive power for, or absorbing it from, the transmission system. These static var compensators have fast response (one to two cycles) to dynamic changes affecting the power flow and, with sufficient VA rating, they can increase both the transient and dynamic stabilities of the power system significantly.
Present static var compensators use fixed and/or thyristor-switched capacitors together with thyristor-controlled reactors. In the capacitive output domain, the fixed and thyristor-switched capacitors approximate, with a positive variance, the vat generation demand (for the desired transmission voltage level) in a step-like manner and the thyristor-controlled reactors absorb the surplus capacitive vars. In the inductive output domain, the thyristor-controlled reactors are operated at the appropriate conduction angle to provide the required var absorption. With proper coordination of the capacitor switching and reactor control, the var output can be varied continuously and rapidly between the capacitive and inductive rating of the equipment. The static var compensator is normally operated to regulate the voltage of the transmission system, sometimes with an option to provide an appropriate voltage modulation to damp power oscillations.
A more recently developed, and radically different, implementation of the static var compensator uses a solid-state switching converter connected in shunt with the transmission line by a coupling transformer. The switching converter is usually a voltage-sourced inverter using gate-turn-off (GTO) thyristors and operated from a dc storage capacitor to generate an output voltage which is in phase with the ac system voltage, V. The amplitude of the inverter output voltage Vo is rapidly controllable with respect to the amplitude of the ac system voltage V. When Vo=V, (ignoring the turn ratio of the coupling transformer) the inverter draws no current. However, when Vo&gt;V, the current drawn by the inverter via the leakage inductance of the transformer is purely capacitive. Similarly, when Vo&lt;V, the current drawn by the inverter becomes inductive. Thus, by controlling the output voltage of the inverter between the rated values of Vomax and Vomin, the reactive output current can be varied continuously from maximum capacitive to maximum inductive.
As stated above, the electric power in a transmission line can also be varied by the control of the overall line impedance. This can be accomplished by providing a controllable series line compensation, which in effect decreases (or increases) the reactive impedance of the line. The thyristor-controlled series line compensator, similarly to the shunt connected static var compensator, can be implemented either by thyristor-switched capacitors or by a fixed series capacitor shunted by a thyristor-controlled reactor.
A novel, solid-state series-compensating scheme, using a switching power converter is proposed in our commonly owned U.S. Pat. No. 5,198,746. In that system, a voltage-sourced inverter is used to insert voltage Vc (of the fundamental ac frequency) in series with the line. Voltage Vc, generated by the inverter, is in quadrature (lagging) with the line current. By making the amplitude of Vc proportional to the amplitude of the line current, the effect of series (capacitive) compensation can be faithfully reproduced.
Rapidly controllable phase shifters have not been implemented in practical systems yet. Schemes, which employ thyristor-controlled, tap-changing transformers, adapting techniques similar to those employed in conventional, mechanically switched tap-changing transformers, have been proposed and evaluated in laboratory models.
In principle, a thyristor-switched tap-changing transformer arrangement can change the magnitude of the voltage added in quadrature to the line voltage by the insertion transformer to control the phase-angle between the sending and receiving end voltages of the transmission line.
The tap-changing transformer type phase-shifter provides a step-like control, although the step size can be minimized by the judicious choice of the turn ratios selected. For example, with three non-identical transformer windings, in proportion of 1:3:9 and a switch arrangement that can bypass a winding or reverse its polarity, a total of 27 steps can be realized.
The thyristor-switched tap-changing transformer arrangement also suffers from the major disadvantage that it cannot generate or absorb reactive power. The reactive power it supplies to or absorbs from the line when it injects the quadrature voltage must be absorbed from it, or supplied to it, by the ac power system. The large voltage drops usually associated with reactive power transfer would tend to negate the effectives of the tap-changing phase-shifter for power flow control in many applications.
A primary object of the present invention is to provide a transmission system power flow controller which can respond rapidly to dynamically control in real time either singly or in combination any one of reactive power, transmission line impedance, transmission line voltage and transmission line voltage angle.